1,3-PARALLEL O/O-INTERACTIONS IN ACYCLIC CARBOHYDRATES - THE CRYSTAL-STRUCTURE AND MOLECULAR-STRUCTURES OF MESO-D-GLYCERO-L-ALTRO-HEPTITOL AND THE MONOHYDRATE, MESO-D-GLYCERO-L-IDO-HEPTITOL, AND MESO-D-GLYCERO-L-ALTRO-HEPTITOL AND D-GLYCERO-L-GALACTO-HEPTITOL HEPTAACETATES

The molecular structures of the title compounds were determined by X-ray crystallography, using direct methods, and refined to final conventional residual parameters of R = 0.042, 0.035, 0.060, 0.062, and 0.047, respectively, meso-D-glycero-L-altro-Heptitol crystallised usually as a monohydrate from moist ethanol with single molecules occurring in the crystal as a racemate of chiral conformers adopting a planar zigzag conformation. Thus, a 1,3-parallel O-3//O-5 interaction is tolerated with a distance of 265.0 pm. By thorough exclusion of water, an unhydrated morph was obtained from ethanol and investigated at 173 K. The conformations of single molecules parallel those observed in the hydrate. The O-3-O-5 distance is 267.6 pm. Molecules of the derived heptaacetate avoid this interaction by bending to a sickle, but tolerate, instead, an O-3//C-6 relationship with a distance of 295.0 pm. Also, in this case, chiral conformers are observed, which are matched by their enantiomers in the unit cell. meso-D-glycero-L-ido-Heptitol is found in a doubly bent chiral conformation which is free of 1,3-parallel interactions and resembles that found in iditol hexaacetate, but not in iditol itself. Again, a racemate of conformers is observed in the unit cell. The molecules of the heptaacetate of chiral D-glycero-L-galacto-heptitol are found in a sickle conformation, free of 1,3-parallel interactions, a situation already observed for the parent alditol but which could not have been predicted from recent investigations of the solid state structures of other alditol acetates. The occurrence of 1,3-parallel O//O interactions in alditols and other acyclic carbohydrates is discussed comprehensively.